Scientists find evidence for early planetary shake-up

Could evidence from a specific binary asteroid pair upset existing planetary theories? ‘The Jupiter trojans, commonly called Trojan asteroids or just Trojans, are a large group of asteroids that share the planet Jupiter’s orbit around the Sun.’ – Wikipedia. There are over a million of these, inhabiting two oval-shaped zones based around what are known as the Lagrangian points L4 and L5 of Jupiter’s orbit (see animation below).

Scientists at Southwest Research Institute (SwRI) studied an unusual pair of asteroids and discovered that their existence points to an early planetary shake-up in our solar system.

These bodies, called Patroclus and Menoetius [see flyby 6 in the graphic], are targets of NASA’s upcoming Lucy mission to the Trojan asteroids. They are around 70 miles wide and orbit around each other as they collectively circle the Sun.

Smaller objects (green) at the Lagrange points each remain in the same relative position – small objects at any other point would be pushed into orbit by gravitational forces [credit: Wikipedia]

They are the only large binary known in the population of ancient bodies referred to as the Trojan asteroids. The two swarms of Trojans orbit at roughly the same distance from the Sun as Jupiter, one swarm orbiting ahead of, and the other trailing, the gas giant.

“The Trojans were likely captured during a dramatic period of dynamic instability when a skirmish between the solar system’s giant planets — Jupiter, Saturn, Uranus and Neptune — occurred,” said SwRI Institute Scientist Dr. David Nesvorny. He is the lead author of the paper, “Evidence for Very Early Migration of the Solar System Planets from the Patroclus-Menoetius Binary Jupiter Trojan,” published in Nature Astronomy.

This shake-up pushed Uranus and Neptune outwards, where they encountered a large primordial population of small bodies thought to be the source of today’s Kuiper Belt objects, which orbit at the edge of the solar system. “Many small bodies of this primordial Kuiper Belt were scattered inwards, and a few of those became trapped as Trojan asteroids.”

A key issue with this solar system evolution model, however, has been when it took place. In this paper, scientists demonstrate that the very existence of the Patroclus-Menoetius pair indicates that the dynamic instability among the giant planets must have occurred within the first 100 million years of the solar system formation.

Recent models of small body formation suggest that these types of binaries are leftovers of the very earliest times of our solar system, when pairs of small bodies could form directly from a collapsing cloud of “pebbles.”

“Observations of today’s Kuiper Belt show that binaries like these were quite common in ancient times,” said Dr. William Bottke, director of SwRI’s Space Studies Department, who coauthored the paper. “Only a few of them now exist within the orbit of Neptune. The question is how to interpret the survivors.”

Had the instability been delayed many hundreds of millions of years, as suggested by some solar system evolution models, collisions within the primordial small-body disk would have disrupted these relatively fragile binaries, leaving none to be captured in the Trojan population.

Earlier dynamical instabilities would have left more binaries intact, increasing the likelihood that at least one would have been captured in the Trojan population. The team created new models that show that the existence of the Patroclus-Menoetius binary strongly indicates an earlier instability.

This early dynamical instability model has important consequences for the terrestrial planets, particularly regarding the origin of large impact craters on the Moon, Mercury and Mars that formed approximately 4 billion years ago. The impactors that made these craters are less likely to have been flung in from the outer regions of the Solar System. This could imply they were made by small-body leftovers of the terrestrial planet formation process.

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The positions of the L3, L4 and L5 Lagrangians also form an equilateral triangle, leading to this…

With Lagrangian points other stable patterns are possible, and so happen. A stable 3:2 resonance pattern of asteroids whose motion gets confined to a basically triangular shape by the combined pull of Jupiter and the Sun. Around Jupiter this group of asteroids is called the Hilda Family, and their route forms a triangle with its three points at the two Lagrange points and at the point on Jupiter’s orbit directly opposite it from the Sun.